Understanding the photodiode, or light sensor, on the Versatile Bio
What it measures:
A photodiode measures light and translates it to a voltage signal that is recorded by the Versatile Bio.
The most common use of a photodiode is as a method of extremely accurate synchronization with visual stimuli, especially when presented on a computer screen.
For example, a photodiode can be connected to the corner of a computer monitor. Alternating visual stimuli events can use a small box at that part of the screen, triggering the photodiode. This would create a marker in the Versatile Bio data corresponding to the change in the stimulus and allow the time synchronization of all other data recorded by the platform. Different intensity of light can be used on the box to denote different trials or parts of a test.
A photodiode can also be used to measure ambient light. This is useful in sleep studies, for example, where it may be useful to know if the lights were on or when sunlight entered the room.
How to use:
The photodiode sensor is placed wherever the light must be measured (i.e. directly on a computer screen).
Connector on Versatile Bio:
The photodiode may pick up the refresh flickering of a computer monitor or florescent lights. To avoid this in the sensor visualization, consider setting a low-pass filter up to 30-40 Hz.
The output voltage roughly corresponds to light intensity recorded by the photodiode sensor.
Wavelength and Temperature Sensitivity
The photodiode is sensitive between 400-700 nm, which generally corresponds to the visual light spectrum. Peak sensitivity is at approximately 550 nm. The device is shielded from the infrared band.
Expect a 2% error for every degree Celsius variation in temperature at the sensor location.
The photodiode sensitivity depends on direction, and it will not see a signal as well that is too peripheral. The following graph details sensitivity at increasing directional angle to the sensor: